1. Field of the Invention
This invention relates to a press apparatus for pressing a moving web formed on a forming wire. More particularly, this invention relates to a press apparatus including a first nip defined by cooperating press rolls and a second nip defined by an extended nip press with a second felt extending through both nips.
2. Information Disclosure Statement
In a press section of a papermaking machine, an important object is to remove as much water from the web as possible without damaging the formation, or crushing the web. It is a well-known fact that with every additional removal of even a fraction of a percent of water from the web, great savings in energy are accomplished. The water which is not removed in the press must be removed thermally in the dryer drum section of the paper machine, and the energy requirements for the removal of a unit of water by evaporation greatly exceed the energy requirements for removal of the same unit of water by pressing.
In U.S. Pat. No. 4,561,939 to E. J. Justus, assigned to Beloit Corporation, there is described a first dewatering nip defined by a pair of cooperating press rolls followed by an extended nip press. This patent discloses a pair of lower felts for transporting the formed web between the first and second nip. However, a suction roll, or the like, is required in order to transfer the formed web from the first lower felt to the second upper felt prior to the web being transported between the second upper felt and the second lower felt. The web, consequently, is not supported by either of the lower felts for a certain portion of the travel between the first and second nips. Also, during suction transfer from one felt to the next, water is transferred back into the sheet from the preceding felt.
The present invention provides means for supporting the web on a lower felt throughout the travel of the web between the first and second nip thereby providing a "no draw press" and eliminating the need for the transfer suction roll of the aforementioned second upper felt of the Justus patent.
Although U.S. Pat. No. 3,023,805 to Walker teaches a single lower felt extending between a first and a second nip, the second nip is not an extended nip press. Furthermore, in an alternative embodiment of the present invention, by the provision of a grooved blanket in the extended nip press, the need for a second upper felt is eliminated.
Therefore, it is a primary objective of the present invention to provide a press apparatus that overcomes the aforementioned inadequacies of the prior art proposals and which provides a press apparatus that contributes a significant improvement to the art of pressing a moving web that has been formed on a forming wire.
Another object of the present invention is the provision of a press apparatus having a first nip defined by a pair of cooperating press rolls and a second nip defined by an extended nip press, the web being supported between these nips by means of a felt which passes through both of these nips.
Another objective of the present invention is the provision of a press apparatus in which the second nip is defined by an extended nip press having a grooved blanket which eliminate the need for having a double felting of the extended nip press.
Another object of the present invention is the provision of a press apparatus in which the lower, second or tandem felt which extends through both the first and the second presses has a void volume which is greater than, and a surface texture that is smoother than the upper felt or felts such that the web tends to stay with the lower felt when the respective felts diverge relative to each other.
Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration of the following description including the claims taken together with the annexed drawings.